A physiologic decline in food intake occurs with advancing age. The physiologic anorexia of aging and its associated weight loss predispose older persons to develop protein-energy malnutrition. In older persons a variety of social and psychologic factors, diseases, and medications can aggravate the physiologic anorexia and lead to severe weight loss. Many of these factors are amenable to treatment, resulting in a reversal of the underlying malnutrition. This article first reviews the physiologic factors responsible for anorexia in older persons. It then reviews the major pathologic processes responsible for producing protein-energy malnutrition in older persons.